The Laotians call it Mae Nam Khong, the Mother of Water. The Vietnamese refer to it as Song Cuu Long, or the Nine Dragons River. The Khmer prefer Tonle Thum, or simply Great River. Whatever its name, the Mekong River is a lifeline for millions in the countries touched by its muddy waters, the world’s largest inland fishery.

On Wednesday, ministers from Vietnam, Thailand, Cambodia and Laos open a three-day meeting in Siem Reap, Cambodia, to decide whether to approve a controversial dam on the mainstream of the river in Laos. Proponents say the dam would bring much-needed revenue to Laos and the region, but scientists warn that it could prove devastating for the environment and the people who depend on the river’s bountiful resources.

About 1,000 species of fish live in the Mekong, making it the second-most biodiverse river in the world, after the Amazon. The river supports fisheries, farming and transportation.

Scientists are concerned that the dam project, known as Xayaburi, will open up a Pandora’s box of dams, given that it is only the first in a cascade of 11 proposed mainstream hydropower plants.

In April, Thailand, Laos, Vietnam and Cambodia — all members of the 1995 Mekong Agreement, which requires prior consultation before any alterations to the river — met on the issue but failed to reach a consensus. Vietnam called for deferring construction of the dam for 10 years, and Cambodia and Thailand expressed concern about the project.

Xayaburi, which is being financed by the Thais, would be the first mainstream Mekong dam in Southeast Asia and would produce about 1,260 megawatts of power, 95 percent of which would be exported to Thailand. Thai building companies were contracted for the $3.7 billion project, and Thailand would operate the dam for 30 years before turning it over to Laos.

Laos has said it hopes to become “the battery of Southeast Asia” in terms of hydropower.

Environmentalists assert that the developers would reap most of the financial benefits, however. Even more important, they say, the countries involved have failed to grasp the full dimensions of the dam’s potential effects on the environment.

“It’s quite clear now that the four countries are not in a position to make an informed decision,” said Ame Trandem, the Southeast Asia program director of the nonprofit group International Rivers. “All scientific evidence points to the fact that this dam will have devastating impacts.”

Zeb Hogan, an aquatic ecologist at the University of Nevada, Reno, said that while “people who have never been to that region might not know it from any other river,” the Mekong is unique in terms of fisheries. Some 2.5 million tons of fish are harvested annually in the basin for a total value of $6.5 billion.

The river is also home to threatened fish, including the critically endangered giant catfish species, which can grow up to 10 feet long. Dr. Hogan and other scientists fear that the dam would drive animals like the giant catfish to extinction.

An estimated 40 to 70 percent of fish stocks in the Mekong, including many on which people depend on for food, are migratory.

A dam would block their annual migrations, meaning that they would not be able to reach spawning grounds to breed. Although developers have proposed things like fish ladders to allow the animals to pass, Dr. Hogan said experience has shown that it is very difficult to build viable fish passages, especially for such a varied mix of large and small species.

A dam would also affect nutrients and flood regimes vital for crop production, scientists say.

“The dam is going to hurt the poorest people because they’re going to lose their livelihoods and food security,” Ms. Trandem said.

The dam’s impact would probably extend 40 to 60 miles north of Xayaburi, where villages would be flooded and people would be forced to relocate, and potentially hundreds of miles south. Currently, no plan exists for moving the approximately 500 households that would be flooded by the dam and there is no guarantee that those families would be compensated for their losses, said David Blake, a doctoral candidate in international development at the University of East Anglia, who spent years in the region studying the dam issue.

“The true situation on the ground is very poorly known or understood because there’s no independent monitoring going on,” he said. Mr. Blake pointed out that in Laos, people can be arrested just for criticizing the government.

The decision-making process has been less than transparent. No public consultations were held in Laos, and only a few sessions were held in Cambodia, Thailand and Vietnam, Ms. Trandem said. No environmental impact assessments were prepared in time to inform the public sessions that were held, she said.

A report issued by the dam’s developer, the CH. Karnchang Public Company, was widely criticized as inadequate and as glossing over environmental and social uncertainties. “I know we can do better than that,” Dr. Hogan said.

Although Thongloun Sisolit, a deputy prime minister of Laos, has said his country will wait for “positive signals” from its neighbors before building the dam, construction workers have already begun clearing the Xayaburi area.

A study commissioned by the United States Agency for International Development examined the potential impacts of all of the region’s proposed dams. There are so many uncertainties that the economic impact could range from a net gain of $33 billion to a net loss of $274 billion, it found.

“If dams go in and we just hope for the best but things go wrong and fisheries collapse, who’s going to pay for that?” said Robert Costanza, a professor of sustainability at Portland State University in Oregon and the lead author of the study. “There’s a lot of uncertainty, and the public should not bear the burden of that uncertainty.”

Dr. Costanza recommends requiring dam builders to post a bond large enough to cover potential damages upfront or asking the international community and neighboring countries to pay Laos not to build any dams. The United States and the World Bank have already stated they support deferring construction.

A decision by the four countries is expected to be announced on Thursday.